984 admin user actions dropdown

[Chelakhovl]

Frontend:
Dropdown Menu in the Дії (Actions) Column:

A dropdown menu was added for each user in the Actions column, containing:

• Send Message (Надіслати лист): Opens a modal for sending a custom email to the user.
• View Profile (Переглянути профіль): Redirects to the detailed profile page of the selected user.
• Block User (Заблокувати користувача): Opens a confirmation modal to block the user.

Send Message:
A form appears for composing a message, which sends the email to the user's registered address.
If successful, a confirmation message is displayed.
On error, an appropriate error message is shown.

View Profile:
Redirects to the user's detailed profile page (/customadmin/users/<user_id>).
Shows an error message if the profile cannot be loaded.

Block User:
Prompts the admin with a confirmation modal before proceeding.
On confirmation, sends a request to block the user.
If successful, updates the user’s status in the UI.
Displays appropriate error messages for inactive users or failed actions.

Error Handling:
Proper error messages are displayed for each action based on the server response.
Buttons are disabled during processing to prevent multiple clicks.

Backend:
Endpoints Implemented:

• Send Message (/api/admin/users/<user_id>/send_message/):
  Validates message data and sends an email to the user.
  Returns appropriate success or error responses.
• Block User (/api/admin/users/<user_id>/block/):
  Blocks the user by marking them as inactive.
  Handles scenarios where the user is already inactive or does not exist.

Tests:
SendMessageView Tests:
Successful message sending.
Error scenarios: short message, unauthorized access, user not found.
BlockUserView Tests:
Successful blocking.
Error scenarios: user already inactive, user not found, unauthorized access.

TASK: #984